1. Field of the Invention
The present invention relates to a method of bonding semiconductor substrates without using an adhesive agent.
2. Description of the Related Art
A wafer direct bonding method has been known as a method of bonding silicon substrates. This method can get rid of a bad influence of thermal strain because an intermediate layer, such as an adhesive agent, does not exist at a bonding interface. Such a method is described in U.S. Pat. No. 4,671,846.
This wafer direct bonding method will be described with reference to FIGS. 7(a)-(c).
Natural oxide layers which are formed on surfaces of mirror-polished silicon wafers, are removed by HF(hydrogen fluoride) to expose silicon atoms on the surfaces. Thereafter, the surfaces are then provided with a hydrophilic property. Namely, silanol groups (Si--OH) are formed on the wafer surfaces by immersing the wafers in an acid solution, such as a mixture of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2, the temperature of which is 90.degree. C. As shown in FIG. 7(a), the surfaces then contact to each other. In this situation, hydroxyl groups which are formed on the surfaces hydrogen-bond. As shown in FIG. 7(b), the wafers are then heated at more than 800.degree. C. in N.sub.2 atmosphere or O.sub.2 atmosphere to strongly bond the two wafers between which Si--O--Si structure is formed. In this case, when the temperature of the heat treatment is more than 1000.degree. C., as shown in FIG. 7(c), Si--Si structure is formed between the two wafers because O(Oxygen) atoms diffuse in the wafers.
According to the above-mentioned method, only Si(silicon), 0 and H(hydrogen) exist at the bonding interface. Therefore, the bonding interface is chemically stable without corrosion or the like. Moreover, when this bonding method is applied the bonding between a sensing portion of a sensor and a stage, it is possible to use the same material of the sensing portion as the material of the stage. Consequently, it is possible to get rid of the problem, such as temperature drift, which is caused by the strain which occurs in accordance with the variation of temperature because it is possible to coincide the thermal expansion coefficient of the sensing portion with that of the stage.
U.S. Pat. No. 4,671,846 says that, when the wafers are heated at more than 300.degree. C., the wafers bond to each other. However, to uniformly bond the whole surfaces of, for example, 3 inches-wafers, the wafers need to be heated at more than 800.degree. C. The reason why is that, when the wafers are heated at the relative low temperature of less than 800.degree. C., many unbonded regions are formed. The unbonded regions are caused by desorption of molecule, such as H.sub.2 or H.sub.2 O, which occurs by dehydration condensation reaction.
For example, when the above-mentioned bonding method is applied to the bond between a sensing portion of a pressure sensor and a stage, the sensor can not withstand high pressure because it is easy to come the sensing portion off the stage owing to the unbonded regions. Moreover, the characteristic of the sensor changes because unwanted stress acts on the sensing portion.
Conventionally, a sensing portion of a pressure sensor and a stage are bonded to each other by anodic bonding method. According to this method, the fracture strength between the sensing portion and the stage is about 1 Kgf/mm.sup.2. In the above-mentioned wafer direct bonding method, the wafers need to be heated at more than 1100.degree. C. so as to make the fracture strength more than 1 Kgf/mm.sup.2.
Moreover, as mentioned above, the wafers need to be immersed in the acid solution to form the silanol groups on the surfaces. Therefore, when this process is carried out after an element is formed on the wafer, the surface to form the element therein needs to be covered with an acid-resisting passivation film to prevent a wiring, made of Al(aluminum), Au(gold) or the like, from corroding away. Consequently, the number of processes increases. The temperature of heat treatment which can be carried out to the wafer having the wiring is at most 450.degree. C. Therefore, it is impossible to get the fracture strength of more than 1 Kgf/mm.sup.2.